I was busy on saturday, so I couldn't polish much. Sadly that includes in-game instructions and tutorials, so here they are:

This is a game of survival. Ten seconds per boss. But it's also about time management: you can be invincible almost as much as you need, but it adds time to the counter.

Three color-coded bosses.
Red slashes and leaves a pit of acid on death.
Blue makes ice blocks and leaves a spinning deathray.
Yellow uses an explosive teleport and leaves a thunderstorm.

You curse each one in turn to die in 10 seconds.

Hold X to channel the curse for faster kills (doubles curse speed)
Hold C to phase and become invincible and speedy (quadruples curse speed in the opposite direction!)

While phased, you add time on your curses and if you add up past 20, you die.

When each boss dies, he leaves a remaining corpse ability and confers all his abilities to the survivors, including their death trigger, so the order you choose to kill them in is important.

Ice blocks won't kill you, but prevent movement unless phased.

Hit escape to return from battle to the boss order screen.

I'm not sure exactly which order is easiest or hardest, but I know killing red first is easier and killing red last is much harder. Even the easiest is probably too hard since I've had no playtesting/balance time. Be warned!

EDIT:
So after playtesting it a bit this evening after the dare, my suggestion for the first run-through is red, blue, yellow. I reckon it's the easiest. I think the opposite lineup is the hardest.

As the bosses die, you'll want to channel less so you don't have to phase as much. Especially if you're doing blue first, you'll have to phase a lot just to get through the beams already. If you are just walking around doing nothing, they all still die on their own!